The invention is based on a valve for controlling fluids in accordance with the type defined in further detail in claim 1.
From European Patent Disclosure EP 0 477 400 A1, a valve which is actuatable via a piezoelectric actuator is already known. This known valve has an arrangement for an adaptive, mechanical tolerance compensation, acting in the stroke direction, for a travel transformer of the piezoelectric actuator, in which the deflection of the piezoelectric actuator is transmitted via a hydraulic chamber. The hydraulic chamber, which functions as a so-called hydraulic booster, encloses a common compensation volume between two pistons defining it, one of which is embodied as an actuating piston with a smaller diameter and is connected to a valve closing member to be triggered, and the other piston is embodied as a control piston with a larger diameter and is connected to the piezoelectric actuator. By way of this compensation volume, tolerances resulting from temperature gradients or different temperature expansion coefficients of the materials used and possible settling effects, can be compensated for without thereby causing any change in the position of the valve closing member.
The hydraulic chamber is fastened between the two pistons in such a way that the actuating piston executes a stroke that is lengthened by the boosting ratio of the piston diameter, when the larger piston is moved by a certain travel distance by means of the piezoelectric actuator. The valve member, pistons and piezoelectric actuator are located on a common axis, one after the other.
A disadvantage in such valves is especially the great structural length, which results from the pistons disposed longitudinally one after the other, and which is a major obstacle when only little installation space is available.
Also in such valves, the leakage losses from the hydraulic chamber along a gap surrounding the control piston or the actuating piston are problematic, since these losses can cause a perceptible loss of efficiency.
The described disadvantages of the known embodiments pertain above all to servo valves for triggering fuel injection valves embodies as common rail injectors, in which high efficiency is desired but only very limited installation space is available.
The valve according to the invention for controlling fluids, as defined by the characteristics of claim 1, having an actuating piston which is disposed in a blind bore of the control piston, and having at least one reducing element to accomplish the boost, advantageously requires only very little installation space.
Furthermore with the valve of the invention, the leakage losses from the hydraulic chamber can be reduced markedly, since far less fluid can escape through the sealing gaps between the control piston, actuating piston and reducing element, which gaps in the embodiment of the invention extend parallel, than via the necessarily larger circumferential faces of a control piston and actuating piston that are disposed serially one after the other.
Because of the low leakage losses, especially at low boosting ratios, better efficiency is achieved. Moreover, a smaller or shorter piezoelectric actuator can be used, which makes it possible to lower the production costs for the valve of the invention markedly, since the dimensioning of the piezoelectric actuator is a significant cost factor.
The boosting ratio is structurally achieved in an especially simple way in the valve of the invention by way of the area ratios between the cross-sectional area of the control piston at the hydraulic chamber, that is, the bottom face of the blind bore, and the cross-sectional area that is composed of the cross section of the actuating piston and the cross section of the at least one reducing element.
In a highly advantageous refinement of the invention, it can be provided that the actuating piston, together with the at least one reducing element, is displaceable for a first portion of its maximum stroke length, and that the actuating piston from the time it reaches the stop executes a remaining stroke length for the at least one reducing element in the bore of the valve body.
This takes into account the finding that while the piezoelectric actuator does furnish a large force reserve as long as the actuator stroke is short, nevertheless the maximum stroke of piezoelectric actuators is also short. With a graduated boost according to the invention, however, it is advantageously possible to bring a major force to bear on the valve closing member for a first portion of the maximum stroke length, since the boosting ratio relative to the control piston is 1:1. Thus the valve closing member can be opened counter to a very high pressure. Once the reducing element has reached its stop, the actuating piston can, depending on the dimensioning, overcome a remaining stroke length with lesser force.
With this kind of embodiment according to the invention of the valve, the piezoelectric actuator can furthermore be reduced still further in size, since to execute the requisite stroke length, the maximum actuator force is now needed for only a small stroke length.
With its embodiment according to the invention, the valve is especially suitable as a servo valve for triggering a fuel injection valve for internal combustion engines, in particular a common rail injector, in which only very limited installation space is available, and in which the servo valve must be opened counter to a high rail pressure, so that a flow specified by an injection needle through the valve seat of the valve closing member is made possible.
Further advantages and advantageous features of the subject of the invention can be learned from the description, drawing and claims.